1. Field of the Invention
The present invention relates to a dielectric filter, a composite dielectric filter, a antenna duplexer, and a communication apparatus incorporating the same, used in high-frequency bands.
2. Description of the Related Art
FIGS. 12A, 12B, 12C, 12D and 12E show a structure of a prior art dielectric filter using a dielectric block mainly used in the microwave band. In this figure, FIG. 12B is a front view in which the dielectric filter is vertically stood, FIG. 12(A) is the upper-surface view, FIG. 12C is the bottom view, FIG. 12D is the left-side view, and FIG. 12E is the right-side view. In this figure, the dielectric block is indicated by 1. Inside the dielectric block 1, resonance-line holes indicated by 2a, 2b, and 2c are disposed, and on the inner surface of the holes, an inner conductor is disposed to form resonance lines 5a, 5b, and 5c. On an outer surface of the dielectric block 1, a ground electrode 3 is formed, and external terminals 6 and 7 separated from the ground electrode 3 are disposed at predetermined positions. The external terminal 6 and the resonance line 5a make a capacitive coupling, whereas the external terminal 7 and the resonance line 5c make a capacitive coupling. In this way, a dielectric filter having a pass-band characteristic of three-stage resonators is formed.
However, in the prior art dielectric filter shown in FIGS. 12A to 12E, the external terminals 6 and 7 perform the input and output of signals in a imbalanced type, in which the respective ground electrodes being used as reference potentials. Therefore, in order to give signals to an amplification circuit of a balance input type for example, it is necessary to convert the signals of imbalance type to those of balance type by using a balun (an imbalance-balance converter). As a result, the filter occupies a large area portion on a circuit board, which being one factor hampering miniaturization.